A number of attachment methods may be utilized to attach and electrically connect a semiconductor die to a package or directly to a substrate or to attach a packaged semiconductor device to a substrate or printed circuit board. Several common methods include wire bonding, solder, adhesive, conductive adhesive, and anisotropic conductive adhesive (ACA). An ACA is a material that enables electrical coupling in one direction (e.g., vertically between a device contact and a substrate contact), but prevents it in other directions (e.g., horizontally between contacts on a device or between contracts on a substrate). ACA may be utilized in various forms, for example a paste, gel, liquid, or film.
There are several approaches to packaging of semiconductor dies. These include mounting the die on a lead frame, either with contacts on the semiconductor die facing up (where the contact pads on the die are typically electrically coupled to the package contacts through wire bonds), or in a flip-chip configuration, in which the contacts on the semiconductor die are facing down and may be electrically coupled to the package contacts more or less directly, for example using solder, conductive adhesive, or ACA. Wire bonding is typically more expensive than flip-chip mounting, and in some configurations may introduce a higher thermal resistance than the flip-chip configuration.
Packages are typically mounted to a substrate or circuit board using solder, for example using a reflow solder process. Semiconductor dies may be directly attached to a substrate or circuit board, without a traditional package, for example using solder or conductive adhesive or ACA. In particular, ACA has been widely used in the attachment of chips to RFID substrates, and has recently been developed for use with LEDs, as detailed in U.S. patent application Ser. No. 13/171,973, filed on Jun. 29, 2011 (the '973 application), U.S. patent application Ser. No. 13/784,417, filed on Mar. 4, 2013, and U.S. patent application Ser. No. 13/949,546, filed on Jul. 24, 2013, the entire disclosure of each of which is incorporated herein by reference.
ACAs have a number of potential advantages over solder, including the ability to use aluminum rather than copper traces, which may result in reduced cost and the ability to manufacture using a roll-to-roll configuration. High-volume tools for RFID manufacture using ACA are commercially available, for example from Muhlbauer in Roding, Germany.
As known in the art, an ACA typically features an adhesive matrix, e.g., an adhesive or epoxy material, containing “particles” (e.g., spheres or particles with other shapes) of a conductive material or of an insulating material coated with a conductive material (such as metal). FIG. 1A depicts a typical schematic of the connection of an electronic device to a substrate via ACA. As shown, an electronic device 105 having multiple contacts 110 has been adhered and electrically connected to conductive traces 160 disposed over a substrate 120 of a circuit board 165 (circuit board 165 includes substrate 120 and conductive traces 160; in some embodiments circuit board 165 may also include one or more components 105 and other elements) via an ACA 130. ACA 130 is typically composed of an adhesive matrix 140 containing a dispersion of particles 150 that are at least partially conductive. In some configurations, the use of ACA may require the use of stud bumps 170; however, these may not be required, for example as shown in FIG. 1B and described in the '973 application. It should be noted that other techniques involving ACAs are possible, and the present invention is not limited by the particular mode of operation of the ACA.
Most ACAs are pressure-activated, and thus require application of pressure and temperature to cure the ACA, forming a permanent electrical and/or mechanical connection. The temperature and pressure are typically applied using a thermode, which provides a means to apply a force and heat to a desired temperature. In high-volume manufacture, multiple thermodes are activated simultaneously over a portion of the substrate, curing multiple electronic devices simultaneously. FIG. 1C shows a schematic of a thermode configuration that includes bottom and top thermodes 181 and 182 respectively. The thermodes typically apply pressure and heat to device 183 directly in the case of thermode 182 and through substrate 180 (substrate 180 may also include conductive traces and other elements, not shown for clarity in FIG. 1C, that may affect or hinder the flow of heat to the ACA) in the case of thermode 181. One issue with this approach is that the thermode field must be reconfigured for each new design—that is, the thermodes must be re-positioned to correspond to the positions of the devices on the substrate. This change-over may be relatively time-consuming and expensive. A second issue is that there is typically a one-to-one correspondence between the number of devices and the number of thermodes required in a given area, i.e., there typically needs to be one thermode for each device within the thermode field area. For systems having very high component counts, this means the curing system must have a very large number of thermodes, which increases cost and configuration time.
A third issue with this approach is that the thermodes have a minimum spacing, determined by the actual thermode size and wiring to the thermodes, which may be larger than the desired spacing between devices on the substrate. This may make it impossible to cure both devices 183 and 185 (see FIG. 1C) simultaneously, because their spacing is less than the minimum thermode spacing. This size limitation may currently be addressed in two ways. The first is to use multiple cure steps, for example curing every other device, to accommodate the thermode size limitations. This is very costly, as it requires repeating the process at least twice, and may also result in damage to the devices and/or substrate during subsequent cure steps. This is particularly onerous for roll-to-roll manufacture, in which the web would have to be run through the tool more than once. A second challenge with this approach is the possibility of pre-curing the ACA in devices adjacent to those being cured. An example of this would be the ACA for device 185 being partially or fully cured without the application of pressure from a thermode, by heat flow through substrate 180 (including the conductive traces not shown in FIG. 1C) from the adjacent thermodes. A second approach is to use a custom thermode system, with fixed position pins that contact each device. Such systems can have a smaller spacing, but require a custom, fixed thermode field for each design, which is expensive and takes a relatively significant time to procure.
In view of the foregoing, a need exists for systems and procedures enabling the low cost reliable curing of pressure-activated adhesives (such as ACAs) for various electronic devices, for example packaged devices and semiconductor dies, directly to the electrical traces of a package or to electrical traces on a substrate without the size and throughput limitations of conventional curing systems.